Environmentalists often call oil from Canada's tar sands the dirtiest fuel on Earth, because the complex process of extracting it spews huge amounts of carbon dioxide into the air.

But by that standard, some of the crude oil pumped in California is just as dirty. In a few cases, it's even worse.

Several California oil fields produce just as much carbon dioxide per barrel of oil as the tar sands do, state data show. A handful of fields yield even more.

All of them are fields that have been pumped for years and now need injections of steam to squeeze out more oil. Power plants create the steam, releasing greenhouse gases in the process. The gases build up in the atmosphere, slowly warming the globe.

In the past, few people knew or cared about the "carbon intensity" of California crude. Now, however, that intensity is helping fuel the fight over a key California policy to combat global warming.

The state's "low carbon fuel standard" requires fuel producers to lower the carbon intensity of the products they sell here 10 percent by 2020. To comply, oil companies will probably have to blend more advanced biofuels into their gasoline and diesel.

But California refineries might also have to stop using some of the crude pumped here in the Golden State, according to an industry trade group.

That carbon-intensive oil would be exported abroad, while the state's refineries would import more low-carbon oil to take its place. And since both the imports and exports would travel in ships - ships burning fuel and releasing carbon dioxide - the added maritime traffic could increase greenhouse gas emissions rather than cut them. The policy, in other words, could backfire.

"We call it crude oil shuffling," said Gina Grey, vice president of strategic policy for the Western States Petroleum Association. "What's the effect if we ship California crude oil to other countries and import low-carbon crude oil instead? Actually, that could increase emissions."

Supporters of the fuel standard point out that most California oil fields aren't as carbon intensive as the tar sands. And even at fields that require steam injection, oil companies have ways to lower carbon intensity by making their operations more efficient. Some companies - including Chevron Corp., which opposes the fuel standard - are even experimenting with using solar power to generate the steam.

Dirty oil sources

At the same time, environmentalists who back the fuel standard say California and the country as a whole need to pay more attention to the carbon intensity of different oil sources. Those sources that are dirtier than others, they say, should be discouraged for the sake of the planet.

The issue lies at the center of the ongoing struggle over the Keystone pipeline extension, which would bring synthetic oil from the tar sands to America's Gulf Coast refineries. It's also being studied by other states - such as Oregon and Washington -that are considering fuel standards of their own.

"There shouldn't be an advantage given to domestic oil that's high-carbon," said Michael Marx, director of the Sierra Club's Beyond Oil campaign. "That's not to say we should lower our standards for the tar sands. We should raise our standards for domestic oil."

Extracting oil from the tar sands is a difficult, multistep process that requires a lot of energy - so much that some companies have proposed building nuclear reactors to supply it. The sand is strip mined and mixed with warm water to separate hydrocarbons from the sand grains. The hydrocarbons are then processed into a kind of synthetic crude that can be piped to refineries.

California oil fields don't require the same treatment. But after more than a century of production, many do need other forms of "enhanced oil recovery" to keep the crude flowing.

The Midway-Sunset field, which runs along Highway 33 in the southwestern San Joaquin Valley, was discovered in 1894 and remains California's largest field. But now, the companies working there rely on injections of steam - more than 200,000 barrels of steam per day - to coax the remaining oil into their wells.

The power plants generating that steam boost the carbon footprint of Midway oil.

Carbon intensity score

The state agency that developed the low carbon fuel standard assigns a carbon intensity score to different sources of crude, a score developed by studying how the oil is extracted from the ground and shipped to refineries. Most oil sources worldwide have scores between 5 and 13, under the system used by California Air Resources Board.

Midway-Sunset oil scores 21.18. Crude from nearby Coalinga scores 25.36. The San Ardo field, whose bobbing pump jacks push up against Highway 101 between King City and San Luis Obispo, scores 28.82.

The high scores for tar sands crude are a source of friction between Californian and Canadian government officials. The Canadians consider the low carbon fuel standard a form of discrimination against one of their exports, even though very little tar sands oil makes its way to California.

To the Air Resources Board, however, the fact that many California oil fields also get high carbon intensity scores proves that the system is fair.

"They like to say we discriminate against out-of-state oil, and as you can see, we don't," said Dave Clegern, spokesman for the Air Resources Board.

Not all California oil fields have high scores. Most rate below 15, with many below 10. The lowest carbon intensity score goes to the Beta offshore oil field south of Long Beach, which rates 1.74.

Oil companies have been pushing for more than a year to have the standard modified or scrapped, so far to no avail. It remains to be seen whether predictions of crude oil shuffling will bear out.

Also unknown: the fuel standard's potential impact on fracking in California.

Dramatic renaissance

The U.S. oil industry is undergoing a dramatic renaissance, as the combination of hydraulic fracturing and horizontal drilling taps large oil and natural gas deposits trapped in shale rock. Domestic oil production has jumped 28 percent since 2008.

Now the industry is eyeing the Monterey Shale, an immense formation below the San Joaquin Valley that could hold 15.4 billion barrels of oil. Companies have tried fracking the formation, although production has yet to take off.

As a result, the Air Resource Board hasn't developed a carbon intensity score specifically for fracked fields in California, Clegern said.